Soil heterogeneity affects the spatial distribution of microbial communities and soil biogeochemical processes. To understand how this heterogeneity affects microbially-driven biogeochemistry, we first examined the spatial heterogeneity of soil moisture, soil carbon (C), and soil nitrogen (N) in relation to local-scale topography. Based on previous studies, we hypothesized that soil moisture varies with topography, with wetter soils in concave areas (troughs) and drier soils in convex areas (ridges), and that soil C:N varies according to the litter quality of nearby plants. We then examined the effects of this heterogeneity on soil biogeochemistry. We hypothesized that enzyme activity varies with plant species and net N mineralization varies with soil moisture. To test these hypotheses, we sampled soils along a topographic gradient from the base of three focal plant species (Prestoea montana, Casearia arborea, and Dacryodes excelsa) in a 16 hectare forest plot in northeastern Puerto Rico. From each soil, we measured hydrolytic enzyme activity, net nitrogen mineralization, soil C:N, and soil moisture. We then analyzed enzymatic activity and net nitrogen mineralization as functions of soil moisture, focal plant species, and soil C:N using linear models.
Results/Conclusions
Topography and plant species captured a range of soil heterogeneity in our study. Soil moisture was significantly higher in troughs than on ridges (p < 0.001) and differed between tree species (p < 0.05). Soil N also varied significantly between species (p < 0.05); however, soil C:N was independent of both species and topography. Extracellular enzyme activity for two cellulose and one hemi-cellulose degrading enzymes (beta-glucosidase, beta-cellobiosidase, and beta-glucosaminidase) varied most significantly by focal species and increased with soil moisture (R2 of 0.35, 0.15, and 0.17, respectively). Soil ammonium was positively correlated with soil moisture and with soil C:N but did not show differences by focal species. Our results suggest that understanding spatial patterns of soil heterogeneity via topography and plant species differences can provide insight into the spatial heterogeneity of biogeochemical process. Further, plant species effects on some biogeochemical processes (e.g. enzyme activity) may exceed the effects of soil properties (e.g. soil moisture) and these processes must be studied in the context of species.